Method for neutralising a stream of hydrocarbon fluid

ABSTRACT

In a process for deacidifying a fluid hydrocarbon stream which comprises carbon dioxide (CO 2 ) and/or other acid gases as impurities, the fluid stream is brought into intimate contact with an absorption liquid in an absorption or extraction zone ( 12 ), the substantially purified fluid stream and the absorption liquid which is loaded with CO 2  and/or other acid gases are separated from one another, and the absorption liquid is subsequently regenerated and then again fed to the absorption extraction zone ( 12 ). To regenerate the absorption liquid, the loaded absorption liquid is first expanded in a first low-pressure expansion stage ( 22 ) to a pressure of from 1 to 2 bar (absolute). The partially regenerated absorption liquid is then heated in a heat exchanger ( 20 ) and then, in a second low-pressure expansion stage ( 29 ), again expanded to a pressure of from 1 to 2 bar (absolute).

The present invention relates to a process for deacidifying a fluidhydrocarbon stream which comprises carbon dioxide (CO₂) and/or otheracid gases as impurities, in which the fluid stream is brought intointimate contact with an absorption liquid in an absorption orextraction zone, the substantially purified fluid stream and theabsorption liquid which is loaded with CO₂ and/or other acid gases areseparated from one another, and the absorption liquid is subsequentlyregenerated and then again fed to the absorption or extraction zone.

In numerous processes in the chemical industry, fluid streams occurwhich comprise acid gases, for example CO₂, H₂S, SO₂, CS₂, HCN, COS ormercaptans, as impurities. These fluid streams can be, for example, gasstreams (such as natural gas, synthesis gas from heavy oil or heavyresidues, refinery gas or reaction gases formed in the partial oxidationof organic materials, for example coal or petroleum), or liquid orliquefied hydrocarbon streams (such as LPG (liquefied petroleum gas) orNGL (natural gas liquids)). Before these fluids can be transported orfurther processed, the acid gas content of the fluid must be markedlydecreased. CO₂, for example, must be removed from natural gas, since ahigh concentration of CO₂ reduces the heating value of the gas. Inaddition, CO₂, combined with water which is frequently entrained in thefluid streams, can lead to corrosion on pipes and fittings.

The removal of sulfur compounds from these fluid streams is ofparticular importance for various reasons. For example, the content ofsulfur compounds in natural gas must be reduced directly at the naturalgas source by suitable treatment measures, since the sulfur compoundsalso form acids in the water frequently entrained by the natural gas,which acids are corrosive. To transport the natural gas in a pipeline,therefore preset limit values of the sulfurous impurities must becomplied with. In addition, numerous sulfur compounds, even at lowconcentrations, are foul smelling and, especially hydrogen sulfide(H₂S), toxic.

Therefore, numerous processes for removing acid gas constituents fromfluid streams such as hydrocarbon gases, LPG or NGL have already beendeveloped. In the most widespread processes, the acid-gas-containingfluid mixture is brought into contact with an organic solvent or anaqueous solution of an organic solvent in a gas scrubber or aliquid/liquid extraction.

An extensive patent literature also exists on such scrubbing processesand corresponding absorption solutions used in these processes. Inprinciple, a differentiation can be made here between two differenttypes of absorption media or solvents:

Firstly what are termed physical solvents are used, which are based on aphysical absorption process. Typical physical solvents arecyclotetramethylene sulfone (sulfolane) and its derivatives, aliphaticacid amides, NMP (N-methylpyrrolidone), N-alkylated pyrrolidones andcorresponding piperidones, methanol and mixtures of dialkyl ethers ofpolyethylene glycols (Selexol®, Union Carbide, Danbury, Conn., USA).

Secondly, chemical solvents are used, whose mode of action is based onchemical reactions in which the acid gases are converted into moreeasily removable compounds. For example, in the case of the aqueoussolutions of alkanolamines most widely used as chemical solvents on anindustrial scale, salts are formed when acid gases are passed through.The alkanolamine solution can be regenerated by heating or stripping, inwhich case the acid gas salts are thermally decomposed and/or strippedoff by steam. After the regeneration process, the amine solution can bereused. Preferred alkanolamines used in the removal of acid gasimpurities from hydrocarbon gas streams comprise monoethanolamine (MEA),diethanolamine (DEA), triethanolamine (TEA), diisopropylamine (DIPA),aminoethoxyethanol (AEE) and methyldiethanolamine (MDEA).

Primary and secondary alkanolamines are suitable in particular for gasscrubbers in which the purified gas must have a very low CO₂ content(for example 10 ppm_(v) of CO₂). The primary and secondary alkanolaminesreact directly with carbon dioxide, forming soluble carbamate. In theaqueous amine solution the carbamate is in a characteristic equilibriumwith bicarbonate. To regenerate the amine solution, on an industrialscale, frequently a two-stage regeneration process is used, the loadedsolvent first being expanded in one or more flash columns, so that aportion of the absorbed CO₂ evaporates from the solution. Residualcarbon dioxide and/or other absorbed acid gases are then removed bystripping with steam. Solvents which comprise primary and secondaryalkanolamines require, however, a large amount of steam to decompose thecarbamate and correspondingly a lot of heat energy.

European patent application EP-A 0 322 924 discloses using an aqueousamine solution which comprises tertiary alkanolamines, in particularMDEA, for deacidifying gas streams. In contrast to primary and secondaryalkanolamines, tertiary alkanolamines do not react directly with carbondioxide, since the amine is completely substituted. Rather, carbondioxide is reacted in a slow reaction with the tertiary alkanolamine andwith water to give bicarbonate. Tertiary amines are therefore suitablein particular for selective removal of H₂S from gas mixtures whichcomprise H₂S and CO₂. Because of the slow reaction of carbon dioxide,the scrubbing process with tertiary alkanolamine solutions must becarried out using a high liquid/gas ratio at a correspondingly highsolvent recirculation rate. Therefore, attempts have been made toincrease the absorption rate of carbon dioxide in aqueous solutions oftertiary alkanolamines by adding further compounds which are termedactivators or promoters (DE-A-15 42 415, DE-A-1 094 428, EP-A-0 160203).

U.S. Pat. No. 4,336,233 describes one of the currently most effectiveabsorption liquids for removing CO₂ and H₂S from a gas stream. This isan aqueous solution of methyldiethanolamine (MDEA) and piperazine asabsorption accelerator or activator (aMDEA®, BASF AG, Ludwigshafen). Theabsorption liquid described there contains from 1.5 to 4.5 mol/l ofmethyldiethanolamine (MDEA) and from 0.05 to 0.8 mol/l, preferably up to0.4 mol/l, of piperazine. The removal of CO₂ and H₂S using MDEA isfurther described in more detail in the following patents of theapplicant: U.S. Pat. No. 4,551,158; U.S. Pat. No. 4,553,984; U.S. Pat.No. 4,537,753; U.S. Pat. No. 4,999,031; CA 1 291 321 and CA 1 295 810(EP 202 600), EP-A-190 434 (CA 1,290,553), EP-A-159 495, EP-A-359 991(U.S. Pat. No. 4,999,031) and WO 00/00271

Since no direct bond is formed between tertiary alkanolamines and carbondioxide, the amine solution can be regenerated very economically. Inmany cases flash regeneration with one or more expansion stages issufficient. An optional additional thermal regeneration requiressubstantially less energy than in the case of solutions of primary orsecondary alkanolamines.

Customarily, the loaded absorption liquid is expanded from a pressureprevailing in the absorption column of 10-100 bar to a pressure of 1-2bar in a low-pressure expansion chamber or low-pressure expansion tower.U.S. Pat. No. 4,336,233 and U.S. Pat. No. 4,537,753 describe a variantof this process in which the loaded absorption liquid is expanded in afirst expansion stage to a pressure of 5 bar or more and in a secondexpansion stage to a pressure of from 1 to 3 bar. In this case the firstexpansion stage is termed a medium-pressure expansion stage(high-pressure flash). Between the bottom of the absorption column andthe inlet of the first expansion chamber, an expansion turbine canadditionally be disposed. EP-A-0 107 783 also describes a multistageexpansion process for regenerating the absorption liquid. In a firstexpansion stage the absorption liquid is expanded to a pressure of morethan 5 bar and in a downstream second stage is expanded to a pressure offrom 1 to 2 bar. In these processes, upstream of the low-pressureexpansion stage, or, if a medium-pressure expansion stage is provided,between the medium-pressure and low-pressure expansion stage, a heatexchanger is disposed for heating the absorption liquid. A disadvantagein these processes is the fact that the absorption liquid coming fromthe bottom of the absorption tower or from the bottom of themedium-pressure expansion stage has a relatively high temperature offrom 80 to 100° C. The energy input in the heat exchanger thereforetakes places at a relatively high energy level.

According to another process described in the literature, the absorptionliquid is regenerated in a multistage expansion process, the lastexpansion stage being operated at reduced pressure compared withatmospheric pressure, typically at a pressure of from 0.5 to 0.8 bar(absolute) (EP-A-0 121 109, EP-A-0 159 495, CA 1,295,810, CA 1,290,553).This process also has economic disadvantages, since in industrial usehigh compressor ratings are required of the pumps used to generate thevacuum and operating the pumps is a further cost factor.

It is an object of the present invention, therefore, to provide animproved process for deacidifying a fluid hydrocarbon stream in whichthe absorption liquid can be economically regenerated.

We have found that this object is achieved by the process according toclaim 1. The present invention therefore relates to a process fordeacidifying a fluid hydrocarbon stream which comprises CO₂ and/or otheracid gases as impurities in which the fluid stream is brought intointimate contact with an absorption liquid in an absorption orextraction zone, so that CO₂ and other acid gases present in the fluidstream are absorbed by the absorption liquid, the substantially purifiedfluid stream and the absorption liquid which is loaded with CO₂ and/orother acid gases are separated from one another, and the absorptionliquid is subsequently regenerated and then again fed to the absorptionor extraction zone. According to the invention, the absorption liquid isregenerated by expanding the loaded absorption liquid to a pressure offrom 1 to 2 bar (absolute) in a first low-pressure expansion stage, sothat a portion of the gases present in the absorption liquid canevaporate, then heating the partially regenerated absorption liquid andthen expanding to a pressure of from 1 to 2 bar (absolute) in a secondlow-pressure expansion stage.

According to the invention, therefore, it is proposed that theabsorption liquid is expanded in two separate expansion steps, in eachcase to a pressure which is only slightly above atmospheric pressure andis preferably from 1.1 to 1.5 bar (absolute). Particularlyadvantageously, the pressure in both low-pressure expansion stages isessentially the same. Between the first and second low-pressureexpansion stages the absorption liquid is heated in order to compensateat least partially for the energy loss occurring in the first expansionstage. Since expansion is performed to a pressure of from 1 to 2 bar assoon as in the first low-pressure expansion stage, the absorption liquidbefore heating is cooler than in the processes known from the prior art.Typically, the heating is performed in a heat exchanger at an absorptionliquid inlet temperature of from 60 to 85° C. The heat energy istherefore fed at a lower energy level, which leads to an improvedoverall efficiency. In previously known processes in which theabsorption liquid is heated before entry to the low-pressure expansionstage, heating of the entire mass stream is necessary, that is to say ofthe absorption liquid and virtually all of the absorbed acid gas. Incontrast thereto, in the inventive process, the acid gas alreadyreleased in the first low-pressure extension stage need no longer beheated.

Advantageously, for the expansion, expansion vessels or expansionchambers are used which can also be constructed as columns. Theexpansion vessels can be free from special internals. However,internals, for example columns equipped with packings, can also be used.When an expansion column is used, the partially regenerated absorptionliquid is taken off at the bottom of the first column and passed intothe heat exchanger. The heated absorption liquid is then passed into thetop region, preferably into the upper third, of the expansion column ofthe second expansion column. From the bottom of the second expansioncolumn, the regenerated absorption liquid is passed back to the top ofthe absorption column. Before entry into the absorption column, anadditional heat exchanger can be disposed, which cools the absorptionliquid in order to increase the loading capacity with acid gases in thesubsequent scrubbing process.

The heating of the partially regenerated absorption liquid after passagethrough the first low-pressure expansion stage and the expansion in thesecond low-pressure expansion stage can also be effected by an expansiondevice and a separate heat exchanger. Heating and second expansion,however, can also be carried out in an integrated device, for example ina heat exchanger equipped with degassing. Particularly preferably, ahorizontal heat exchanger is used, in which the heating of theabsorption liquid and the release of the acid gases in the secondexpansion process can be performed simultaneously, for example ahorizontal thermosyphon. The absorption liquid outlet temperature isthen advantageously below the boiling temperature of the lowest-boilingcomponent of the unloaded absorption liquid at a given operatingpressure.

The gas released in the two expansion stages consists chiefly of CO₂.Entrained water and residues of amines can be fed back via refluxcondensers to the absorption medium circuit or the absorption column.The gases taken off at the top of the first and second low-pressureexpansion stages are therefore advantageously passed via condensers, forexample reflux condensers. A separate reflux condenser can be assignedhere to each low-pressure expansion stage. Preferably, however, thegases released in the first and second expansion stages are taken offvia a shared condenser.

If a heat exchanger equipped with degassing is used in the secondexpansion stage, the gas released can also be returned to the bottom ofthe first expansion stage.

The gas taken off via the condensers, after any adsorption of residualtraces of amine has been carried out, consists of high-purity foodquality CO₂, which can be further processed accordingly.

According to a variant of the inventive process, at least onemedium-pressure expansion stage (high-pressure flash) can be providedupstream of the first low-pressure expansion stage, in whichmedium-pressure expansion stage the loaded absorption liquid which istaken off from the absorber bottom is first expanded to a pressure ofgreater than or equal to 3 bar (absolute), typically from 5 to 8 bar(absolute).

A particular advantage of the inventive process is that CO₂ and also arelatively large amount of other acid gases, such as H₂S, can be removedto a sufficient extent from the absorption liquid using the twolow-pressure expansion stages proposed according to the invention, sothat the absorption liquid is regenerated without further thermaltreatment, that is to say without any additional strippers (strippingcolumns). Depending on the contamination of the treated fluidhydrocarbon stream with other acid gases, however, it can beadvantageous to regenerate the absorption liquid after expansion using astripper. For example, other acid gases present in the absorptionliquid, such as H₂S or COS, and residues of CO₂, can be removed bystripping with steam or nitrogen.

The inventive process can be carried out using the most variedabsorption liquids, aqueous amine solutions which comprise at least oneamine being preferred as absorption liquids. Particularly preferably,alkanolamines, such as monoethanolamine, diethanolamine,triethanolamine, diisopropanolamine, aminoethoxyethanol, etc., are used.Very particularly preferably, the absorption liquid comprisesmethyldiethanolamine (MDEA), dimethylethanolamine or piperazine, asindividual components or as a mixture of two or three of thesecomponents.

Generally, from 1.5 to 6 mol of amine are used per l of absorptionliquid, preferably from 2 to 5 mol/l, and in particular from 3.0 to 4.5mol/l.

When a tertiary amine is used, an activator, in particular piperazine ora derivative thereof, can be used. The activator is generally used in anamount of from 0.05 to 3 mol, in particular from 0.1 to 2 mol, per l ofabsorption liquid.

Particularly preferably, methyldiethanolamine is used together withpiperazine. The amount of methyldiethanolamine in this case is from 30to 70% by weight, in particular from 35 to 60% by weight, or from 40 to60% by weight, and particularly preferably from 45 to 55% by weight,based on the weight of absorption liquid. The amount of piperazine issuch that the weight ratio of methyldiethanolamine to piperazine is from9 to 15, preferably from 11 to 15.

The present invention is described in more detail below with referenceto an example shown in the accompanying drawings. The example relates toa gas scrubber. Regeneration of absorption liquid used in theliquid/liquid extraction (for instance in an LPG scrubber) proceedscorrespondingly, however.

In the drawings:

FIG. 1 shows a diagrammatic representation of the process sequence of apreferred embodiment of the inventive gas scrubber;

FIG. 2 shows a detail of a variant of the process of FIG. 1;

FIG. 3 shows a second preferred embodiment of the inventive process;

FIG. 4 shows a detail of a variant of the process of FIG. 3;

FIG. 5 shows a diagrammatic representation of a heat exchanger operatingby the thermosyphon principle; and

FIG. 6 shows a diagrammatic representation of the process sequence of agas scrubber according to the prior art.

First, with reference to FIG. 6, the principle of the process sequenceof a gas scrubber according to the prior art is explained. A fluidmixture, which can contain, for example, natural gas as product ofvalue, and in addition comprises CO₂ and, if appropriate, other acidgases such as H₂S or COS, is passed via a feed line 10 to an absorptioncolumn 11. Before entry into the absorption column, separation devices(not shown) can be provided which remove, for example, liquid dropletsfrom the crude gas. The absorption column 11 has an absorption zone 12,in which intimate contact is ensured of the acid crude gas with anabsorption liquid low in acid gases, which passes via a feed line 13 tothe top region of the absorption column 11 and is conducted incountercurrent to the gas to be treated. The absorption zone 12 can beimplemented, for example, by plates, for instance sieve plates of bubblecap plates, or by packings. Typically, from 20 to 34 plates are used. Inthe top region of the absorption column 11, from 1 to 5 backwash plates14 can be disposed, in order to decrease the loss of easily volatileconstituents of the absorption liquid. The backwash plates 14constructed, for example, as bubble cap plates are fed via a condensateline 15 with water, through which the treated gas is passed. The gasstream which is substantially freed from acid gas constituents leavesthe absorption column 11 via a top takeoff 16. In the line 16, aseparator (not shown) can be disposed, in particular if no backwashplates are provided in the column 11, which separator removes entrainedabsorption liquid from the gas stream.

Instead of the single-stage absorption device described here, atwo-stage variant can also be used, as depicted, for example, in FIG. 2of U.S. Pat. No. 4,336,233.

In the absorption column there typically prevails a pressure of from 1to 120 bar (absolute), preferably from 10 to 100 bar. The absorptionliquid is passed into the column top at a temperature of from 40 to 70°C. and taken off at the column bottom at from 50 to 100° C.

The acid-gas-containing absorption liquid leaves the absorption column11 via a line 17 and passes via an optionally provided expansion turbine18 and a line 19 into a heat exchanger 20, in which the absorptionliquid is heated by from 5 to 30° C. Between the adsorption column 11and the heat exchanger 20, or between the expansion turbine 18 and theheat exchanger 20, there can be disposed one or more medium-pressureexpansion columns (not shown here), in which the absorption liquid isexpanded from a higher pressure to a lower pressure of typically morethan 3 bar (absolute). The medium-pressure expansion columns serve,primarily, not for regenerating the absorption liquid, but for higherpurity of the acid gas, which is achieved via an upstream release ofinert gases in the medium-pressure expansion columns. An example of agas scrubber in which the absorption liquid is regenerated in amedium-pressure expansion column and a downstream low-pressure expansioncolumn is described, for example, in U.S. Pat. No. 4,537,753.

The loaded absorption liquid is fed via a line 21 into the top region ofan expansion column 22. In the example shown, at the top of theexpansion column 22 there is provided a heat exchanger with topdistributor or condenser 23 which feeds back entrained droplets of theabsorption liquid to the expansion column. Via the line 24, releasedacid gas is taken off which, if appropriate after removing residualtraces of amine, is present as high-purity gas and can be used, forexample in the absence of sulfur compounds, as pure CO₂ (food grade) inthe food industry. The regenerated absorption liquid leaves theexpansion column 22 via a line 25 and is fed back via a pump (not shown)and a heat exchanger 26 which may optionally be provided to cool theabsorption liquid, via the line 13, to the top region of the absorptioncolumn 11. Between the expansion column 22 and the absorption column 11,one or more strippers (stripping columns, not shown here) can also bedisposed, in which the absorption liquid is conducted in countercurrentto a gas stream, for example steam or nitrogen, in order to removeresidual acid gas constituents from the absorption liquid.

Referring to FIG. 1, the process sequence of a first variant of theinventive gas scrubbing process is now explained in more detail. Theactual gas scrubbing corresponds to the process which is known per se,already explained in connection with FIG. 6, where in FIG. 1 elementswhich correspond to those already described in FIG. 6 are given the samereference numbers. Acid-gas-containing crude gas is again passed via aline 10 into the absorption column 11 where it is brought into intimatecontact with a absorption liquid. The absorption liquid is taken off viathe line 17 in the bottom of the absorption column 11 and passed via anoptionally present expansion turbine 18 to the top region of a firstlow-pressure expansion column 22. In the expansion column 22, theabsorption liquid is expanded to a pressure which essentiallycorresponds to atmospheric pressure or which is only slightly, about 1bar, above it. The absorption liquid is not heated before entry into thefirst absorption column. The gases released during the expansionprocess, in particular CO₂, are taken off from the top region of thecolumn via the condenser 23 and the line 24. The partially regeneratedabsorption liquid passes via a line 27 into a heat exchanger 20 and isheated there before it flows via a line 28 into the top region of asecond low-pressure expansion column 29. In the second column 29, theabsorption liquid is again expanded to a pressure of from 1 to 2 bar(absolute), with further gas fractions being able to evaporate from theabsorption liquid. The gas is again, after passage through a refluxcondenser 30, taken off from the top region of the column 29 via a line31. The regenerated absorption liquid passes via a line 32 and anoptionally provided heat exchanger 26 via the line 13 back into the topregion of the absorption column 11. By means of the heat exchanger 26,the temperature of the absorption liquid can be decreased prior to entryinto the absorption column 11.

FIG. 2 shows a variant of the process shown in FIG. 1. Instead of thereflux condensers 23,30 provided in FIG. 1 for each expansion column22,29, the gases taken off from the top regions of the columns 22,29 areconducted via lines 33,34 to a shared reflux condenser 35, from whichthey are removed via a line 36.

FIGS. 3 and 4 show a further embodiment of the inventive process inwhich the heating of the absorption liquid and the second expansionstage are implemented by a degassing-equipped heat exchanger 37. In thisthe absorption liquid is firstly heated and secondly acid gas isreleased. The gases released in this case are, in the variant of FIG. 3,passed via a line 38 into a reflux condenser 39 and from there are takenoff via a line 40. According to the variant of FIG. 4, the gasesreleased in the heat exchanger 37 are passed back via a line 41 to thebottom region of the first expansion column 22. In both cases theregenerated absorption liquid passes via the line 42 and an optionallyprovided heat exchanger 26 and the line 13 back to the top region of theabsorption column 11.

The degassing-equipped heat exchanger 37 used in the process variants ofFIGS. 3 and 4 is preferably a horizontal thermosyphon, as shown indetail in FIG. 5 (again the same reference numbers have been used forelements already described in FIGS. 3 and 4). The partially regeneratedabsorption liquid passes via the line 27 into the heat exchanger 37where it is brought into contact with a U-shaped tube 43, through whichflows a heat-exchange medium, and is in part vaporized. In the space 44of the heat exchanger 37 there is thus produced a liquid/vapor mixture.The CO₂ released and residual acid gas constituents are taken off vialine 38, while the regenerated absorption liquid leaves the heatexchanger 37 via line 42.

EXAMPLE

A feed gas consisting of 19% (v/v) CO₂, 1% (v/v) oxygen, 70% (v/v)methane and 10% (v/v) ethane is to be purified by an amine scrubber tothe extent that the residual CO₂ content in the purified gas is 2.5%(v/v).

Amine scrubbing is carried out using a piperazine-activated 40% strengthaqueous MDEA solution as absorption liquid.

In a comparative example, the absorption liquid is regenerated in aprocess according to FIG. 6 (without expansion turbine) using a heatexchanger (rich liquor heater), a low-pressure expansion column withreflux condenser (low-pressure flash) and a cooler for the regeneratedabsorption liquid.

The inventive process using two low-pressure expansion stages is carriedout not only according to the variant of FIG. 1, but also according tothe variant of FIG. 3, in each case no expansion turbines beingprovided.

Experiments with a fixed circulation rate of absorption liquid:

In the variant of FIG. 1, the energy input is 6.4% lower than in theprocess variant of the prior art according to FIG. 6. In the case of theinventive process according to FIG. 4, in which the gas released in theheat exchanger 37 is fed back to the bottom of the first expansioncolumn, the energy savings are as much as 13.5%.

When separate reflux condensers are used, the inventive process produces36% high-purity CO₂ in food grade quality.

Process procedure using a fixed energy input:

If the energy introduced by the heat exchanger 20 is the same in allprocess variants, it is observed that to achieve the requiredspecification of 2.5% residual CO₂, in the process of the prior art, asolvent circulation rate higher by 4.4% and a cooling capacity higher by4.35% are required.

1. A process for deacidifying a fluid hydrocarbon stream which comprisesCO₂ and/or other acid gases as impurities, in which the fluid stream isbrought into intimate contact with an absorption liquid in an absorptionor extraction zone, the substantially purified fluid stream and theabsorption liquid loaded with CO₂ and/or other acid gases are separatedfrom one another, and the absorption liquid is subsequently regeneratedand then again fed to the absorption or extraction zone, whichcomprises, to regenerate the absorption liquid, (a) expanding the loadedabsorption liquid in a first low-pressure expansion stage to a pressureof from 1.1 to 2 bar (absolute), (b) heating the partially regeneratedabsorption liquid in the first low-pressure expansion stage and liquidin the first low-pressure expansion stage, and (c) again expanding thepartially regenerated absorption liquid in a second low-pressureexpansion stage to a pressure of from 1.1 to 2 bar (absolute), whereinthe pressure in (a) is essentially the same as the pressure in (c).
 2. Aprocess as claimed in claim 1, wherein the absorption liquid is expandedin the first and second low-pressure expansion stages to a pressure offrom 1.1 to 1.5 bar (absolute).
 3. A process as claimed in claim 1,wherein the steps (b) and (c) are carried out in a degassing-equippedheat exchanger.
 4. A process as claimed in claim 1, wherein the gasesreleased in the first and second expansion stages are taken off viaseparate condensers.
 5. A process as claimed in claim 1, wherein thegases released in the first and second expansion stages are taken offvia a shared condenser.
 6. A process as claimed in claim 3, wherein thegases released in the heat exchanger are fed back to the first expansionstage.
 7. A process as claimed in claim 1, wherein the absorption liquidis expanded to a pressure of at least 3 bar in at least onemedium-pressure expansion stage upstream of the first low-pressureexpansion stage.
 8. A process as claimed in claim 1, wherein theabsorption liquid, after the expansion, is regenerated by stripping withnitrogen or steam.
 9. A process as claimed in claim 1, wherein theabsorption liquid used is an aqueous amine solution.
 10. A process asclaimed in claim 9, wherein an absorption liquid is used which comprisesmethyldiethanolamine or dimethylethanolamine or a mixture thereof.
 11. Aprocess as claimed in claim 10, wherein tan absorption liquid is usedwhich additionally comprises piperazine.
 12. A process as claimed inclaim 11, wherein the absorption liquid comprises from 30 to 70% byweight of methyldiethanolamine and piperazine in an amount such that theweight ratio of methyldiethanolamine to piperazine is from 9 to
 15. 13.A process as claimed in claim 12, wherein the absorption liquidcomprises from 35 to 60% by weight of methyldiethanolamine.
 14. Aprocess as claimed in claim 11, wherein the absorption liquid comprisesfrom 45 to 55% by weight of methyldiethanolamine and piperazine in anamount such that the weight ratio of methyldiethanolamine and piperazineis from 11 to
 15. 15. A process as claimed in claim 12, wherein theabsorption liquid comprises from 45 to 55% by weight ofmethyldiethanolamine and from 3 to 4% by weight of piperazine.